1. Field of the Invention
The invention relates to a detection device for an amplifier arrangement comprising means coupled to the input stage to detect during operation an amplitude difference between a signal to be amplified, which is applied to a second input terminal, and a part of the signal appearing on the output terminal, which part of the signal is applied to a first input terminal via a coupling network.
2. Description of the Related Art
In transistor amplifier arrangements, the maximum amplitude of the amplified signal is, in fact, dictated by the value of the supply voltage to which the amplifier arrangement is connected. When the output transistors of the amplifier arrangement are driven into saturation, the amplitude of the amplified signal is limited to the value of the supply voltage and becomes highly distorted.
In order to preclude this undesired distortion, it is common practice to arrange a resistor in the emitter or collector line of one or more of the output transistors. When the current through the relevant output transistor exceeds a specific limit value, the voltage across this resistor turns on an auxiliary transistor, which drains a part of the drive signal of the relevant output transistor and thereby reduces the drive to this output transistor.
The currents which flow in an output transistor of an amplifier arrangement depend on not only the extent to which this transistor is driven but also on the impedance of the load connected to the arrangement. This means that, if the instant at which the amplified signal is limited by the supply voltage is deleted by measuring the current flowing in one or more output transistors, this detection will be load dependent.
This is a drawback, in particular in audio amplifiers which are generally designed for connection to loudspeakers having different impedance values. Since, as is well known, the impedance of a loudspeaker is frequency dependent, the saturation detection on the basis of the currents in an output transistor will also exhibit an undesirable frequency dependence in amplifiers of this type.